Method of heat-treating metal parts with a bright finish



Patented Aug. 2, 1949 METHOD OF HEAT-TREATING METAL PARTS WITH A BRIGHTFINISH Artemas F. Holden, New Haven, Conn.

No Drawing.

Application June 20, 1944,

Serial No. 541,281

4 Claims. 1

The present invention relates to metallurgy and particularly to heattreatment liquid salt baths, more especially for use in heatin andquenching work pieces of steel, similar to austempering but with thevariation that after the pieces of steel have been heated from 1350 F.to 1700 F. and quenched at a temperature of from 400 F. to 800 F., thework is held at this lower temperature for a substantial period of time,dependent upon the character of the metal and the size of the piecescomprising the work, and the work is then transferred to another bathoperating at from 900 F. to 1100 F., and is finally quenched in coldwater.

In accordance with the present invention, the first or high temperatureNo. 1 bath in which the work is submerged, comprises a molten bath ofpotassium chloride 34% to 50%, sodium chloride 45% to 65% and sodiumfluoride to 8%, the preferred percentage being potassium chloride 43%,sodium chloride 55%, and sodium fluoride 2 This No. 1 liquid bath may beoperated from 1350 F. to 1700 F. Since the work pieces are covered withthe molten salts of this bath as they are being transferred to the No. 2bath, the work remains bright at this stage.

The No. 2 bath, or low temperature bath, comprises sodium hydroxide 30%to 70% and potassium hydroxide 30% to 70%; the preferred percentagesbeing sodium hydroxide 50% and potassium hydroxide 50%. This second bathis adapted to be operated from 400 F. to 800 F.

This No. 2 bath is preferably heated in a steel pot, with an electricelectrode furnace using substantially pure nickel electrodes, that is,commercial nickel rods. No. 2 bath, when heated as specified, are of abright finish with no surface oxidation.

It was found that where this No. 2 bath was heated in a steel pot in agas fired furnace, that the work came from the pot with a blue oxidefinish. The reason why the work comes from a gas fired furnace with ablue oxide finish, and comes from the electric electrode furnace with abright non-oxidized finish, may perhaps be due to electricaldecomposition of the salts in the electric furnace, thereby producing asmall amount of metallic alkali metals which act to prevent theformation of any oxide on the surface of the Work. This suggestion ismerely a possible theory and is not put forward as a definiteexplanation, but the fact remains, however, that the results are asstated.

In the course of the operation of the low temperature hydroxide No, 2bath, the hydroxides are Work pieces coming from this graduallytransformed into carbonates. This may be due to the bath taking upcarbon dioxide from the surrounding atmosphere. Also, some of the No. 1high temperature bath, comprising the chlorides and fluorides specified,is brought over into the No. 2 low temperature bath due to the transferof work pieces from the No. 1 bath to the N0. 2 bath. This contaminationof the No. 2 bath by additions from the No. 1 bath and the breakdown ofthe hydroxides to carbonates results in a thickening of the No. 2 bathwhich may continue to such extent that the materials of the No. 2 bathbecome too thick for satisfactory operation at the temperaturesspecified, at which time a portion or all of this bath is removed bybailing out, and is restarted with the proportions of the materials aspreviously herein specified.

After the temperature of the work has been reduced to the temperature ofthe No. 2 bath, it is desirable, in most cases, to again raise thetemperature of the work pieces in a No. 3 bath which is operated from900 F. to 1100 F. and comprises the following chemicals: soda ash 20% to30%, sodium chloride 5% to 15%, sodium cyanide 8% to 12%, sodiumfluoride 10% to 15%, and caustic soda 30% to 50%, the preferred formulabeing soda ash 24%, sodium chloride 10%, sodium cyanide 10%, sodiumfluoride 12%, and caustic soda 4.4%, The work, when being carried fromthe No. 2 bath to the No. 3 bath, carries with it a small percentage ofthe chemicals of theNo. 2 bath which adhere to the work pieces, and,therefore, appropriate adjustments should be made from time to time inthe N0. 3 bath to retain the same at approximately thepercentages'specified, more especially by the addition of sodium cyanideand sodium fluoride.

The preferred percentages in the baths No. 1, No. 2, and No. 3 are thepercentages of the mixtures when the baths are started, and whencontamination reaches a point whereby the bath becomes thickened at thetemperature at which the bath is supposed to be running, then it becomesnecessary to bail out the thickened bath and replace the same with amixture substantially the same as the proper mixture when the bath isstarted.

When the work is removed from the No. 3 bath, it is of a bright finishand is immediately submerged in clear cold water for quenching, and theresult is a clean bright surface thereby eliminating any major picklingor other treatment. The surface of the work is sufiiciently clean, afterleaving the No. 3 bath, so that it may be electrolytically plated withany suitable finishing metal, tinned, brazed, or otherwise treated,without requiring any special treatment to remove oxides such as usuallyform when ferrous parts are heat treated.

The three baths specified are adaptable for treatment of various metals,comprising stainless steels and other steels and non-ferrous metals. Anexample of use of the specified baths with a particular kind of steelcomprises the treatment of SAE 4140 steel. This SAE 4140 steel washeated to'1550" F. in the No. 1 bath, and was quenched to a rangebetween 600 F. and 700 F. in the No. 2 bath, then transferred to the No.3 bath where it was heated to 900 F. and finally quenched in clear coldwater. The physical properties of this SAE 4140 steel after thetreatment, in accordance with the present method, was found to be:reduction in cross-sectional area of from 45% to 59%; and elongation offrom 13% to 16% under the application of a pull of from 160,000 poundsto 170,000 pounds, on a standard a testing machine.

When it is desired to merely anneal the work, the No. 1 and No. 2 bathson]; need to be used, and the No. 2 bath may be run from 600 F. to 1300F.

What I claim is:

1. In the method of heat treating steel parts and providing a non-oxidebright finish thereon, wherein the parts are heated in a first moltensalt bath operated at a temperature of from 1350 F. to 1700 F., theparts being retained in the bath until heated to the temperature of thebath; the parts then being transferred to a second molten salt bathwithout substantial loss of temperature to the parts during saidtransfer, the second bath being operated at a temperature of from 600 F.to 1300 F., the parts being retained in the second bath until they haveacquired the temperature of said bath; the parts then being removed fromthe second bath and. quenched in water: the improvement which comprisesthe first bath being potassium chloride 34% to 50%, sodium chloride 45%to 65% and sodium fluoride to 8% and the second bath being sodiumhydroxide 30% to 70% and potassium hydroxide 30% to 70% heated in anelectrode type electric furnace.

2. The method of heat treating steel parts with a non-oxide brightfinish thereon according to claim 1, and wherein the second bathcomprises substantially 50% sodium hydroxide and 50% potassiumhydroxide.

3. In the method of heat treating steel parts and providin a non-oxidebright finish thereon,

wherein the parts are heated in a first molten salt bath operated at atemperature of from 1350 F. to 1700 F., the parts being retained in thebath until heated to the temperature of the bath; the parts then beingtransferred to a second molten salt bath without substantial loss oftemperature to the parts during said transfer, the second bath beingoperated at a temperature of from 400 F. to 800 F., the parts beingretained in the second bath until they have acquired the temperature ofsaid bath; the parts then being removed from the second bath andtransferred to a third bath, the third bath being operated at .atemperature of from 900 F. to 1100 F., the

parts being retained in the third bath until they have acquired thetemperature of said third bath; the parts then being removed from thethird bath and quenched in water: the improvement which comprises thefirst bath being potassium chloride 34% to 50%, sodium chloride 45% toand sodium fluoride /2 to 8%, the second bath being sodium hydroxide 30%to and potassium hydroxide 30% to 70% heated in an electrode typeelectric furnace, and the third bath consisting substantially of sodaash 24%, sodium chloride 10%, sodium cyanide 10%, sodium fluoride 12%and sodium hydroxide 44%.

4. The method of heat treating steel parts with a non-oxide brightfinish thereon according to claim 3, and wherein the second bathcomprises substantially 50% sodium hydroxide and 50% pctassiumhydroxide.

ARTEMAS F. HOLDEN.

REFERENCES CITED The following referenlces are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,237,434 Holden Apr. 8, 19412,309,745 Bergin Feb. 2, 1943 2,370,959 Holden Mar. 6, 1945 FOREIGNPATENTS Number Country Date 12,816 Great Britain June 7, 1905 OTHERREFERENCES Fuels and Furnaces, pages 1169, 1173-1174, Sept. 1927.

Iron Age, page 721 and advertising page 16, Nov. 10. 1932; ibid.. pages62-69, Feb. 10, 1944,

